The present invention relates to a ground fault circuit interrupter (GFCI) for use in detecting the occurrence of ground fault or like conditions and for thereafter promptly interrupting an electrical connection between an AC electrical power source and load circuit, and more particularly, the present invention relates to a ground fault circuit interrupter which provides reliable and substantially instantaneous detection and clearance of ground faults.
One important function of a ground fault circuit interrupter (GFCI) is to provide electric shock protection to individuals that come into contact, or close proximity, to an unintentionally exposed wire, or to a conductive component of an ungrounded appliance in which an internal wiring fault undesirably provides an electrical connection between the wiring and the ungrounded component. A common example of where such a fault is particularly a problem is at a construction site where workers regularly utilize portable electrical equipment and devices that are electrically connected to an AC power supply or generator.
Ground fault circuit interrupters provide the required protection by de-energizing the electrical connection between the power source and load circuit when an undesired loop connection is detected between the hot lead of the power source and some other circuit which is typically to ground via an individual""s body that forms part of, and completes, the circuit. This type of ground fault generally occurs due to unnoticed faulty wire insulation. If a ground fault circuit interrupter does not interrupt such a fault, fault current can pass through the human body and cause serious physical injury and/or death.
The extent of damage to the human body caused by such a ground fault depends on the relative amount of fault current and the length of time of the application of the current through the body. Ground fault current as small as several milliamps may be capable of causing injury, and the longer the application of fault current through the body, the more severe the injury and the greater the potential for death. At this relatively low level of current, a fuse, circuit breaker, or other overcurrent protection device will not detect the existence of such a potentially dangerous fault. Thus, ground fault circuit interrupters provide an important and potentially life saving function, and it is imperative that ground fault circuit interrupters are reliable and cut off any fault current as quickly as possible.
Examples of various types of ground fault circuit interrupters utilized in various applications are disclosed by U.S. Pat. No. 3,558,980 issued to Florance; U.S. Pat. No. 3,668,470 issued to Ambler et al.; U.S. Pat. No. 4,410,925 issued to Tucker et al.; U.S. Pat. No. 5,774,316 issued to McGary et al.; U.S. Pat. No. 5,835,322 issued to Smith et al.; U.S. Pat. No. 5,844,759 and U.S. Pat. No. 5,943,198 issued to Hirsh et al.; U.S. Pat. No. 5,933,063 issued to Keung et al.; and U.S. Pat. No. 6,278,596 issued to Simpson.
Prior art ground fault circuit interrupters are relatively slow and relatively unreliable. For example, electromechanical relays and solenoids operate relatively slowly after being energized or de-energized, and the mechanical contacts of such relays are unreliable because they can weld together over time. Thus, even if an electro-mechanical relay detects a ground fault, it may open too slowly, or not at all, thereby unnecessarily subjecting an individual to significant harm. Similarly, the use of thyristors as switching components for a GFCI are also too slow. For instance, it is highly desirable for safety reasons that arcing type faults and other faults be detected and cleared in a period of time less than that about 16 milliseconds which corresponds to the time required for a single 60 Hz sine wave. None of the above referenced components or known GFCIs is believed to meet this requirement for all faults occurring anytime during the input AC sine wave.
Other problems are also associated with prior art ground fault circuit interrupters. For example, a failure of any one component in a GFCI may prevent the GFCI from functioning properly and promptly to detect and/or clear a ground fault thereby unnecessarily subjecting individuals to serious injuries. In addition, some GFCIs only detect ground faults looping from the hot power lead and not from both the hot and neutral leads. Further, some GFCIs require the user to reset the GFCI after a non-fault power interruption and thereby permit the GFCI to be in a non-operating condition the instant when the power is re-applied. Still further, prior art GFCIs require heavy integration of the fault signal before ultimately opening the circuit, and thus, cannot clear a ground fault within 16 milliseconds. Finally, some prior art GFCIs require the use of a separate ground return line to sense a fault.
The use of semiconductors as switching elements is known with respect to overcurrent interrupters and devices for detecting overcurrent conditions within a circuit. Examples are disclosed by U.S. Pat. No. 3,060,348 issued to Todd; U.S. Pat. No. 3,178,617 issued to Coker; U.S. Pat. No. 3,302,062 issued to Craig; U.S. Pat. No. 3,303,388 issued to Means; U.S. Pat. No. 3,311,787 issued to Gunderman; U.S. Pat. No. 3,571,608 issued to Hurd; U.S. Pat. No. 3,654,518 issued to Phelps et al.; U.S. Pat. No. 3,678,291 issued to Coe; U.S. Pat. No. 4,031,432 issued to Proctor; U.S. Pat. No. 4,825,330 issued to Walchle; U.S. Pat. No. 4,914,542 issued to Wagoner; U.S. Pat. No. 4,926,288 issued to Bradley; U.S. Pat. No. 5,216,352 issued to Studtmann et al.; U.S. Pat. No. 5,319,515 issued to Pryor et al.; U.S. Pat. No. 5,606,482 issued to Witmer; U.S. Pat. No. 5,926,354 issued to King; U.S. Pat. No. 6,002,566 issued to Arikawa et al.; and U.S. Pat. No. 6,067,219 issued to Armstrong et al.
Although the aforementioned interrupters may function satisfactorily for their intended purposes, there exists a need for a ground fault circuit interrupter that is highly reliable throughout its life and that provides substantially instantaneous switching (ie., detects and clears a ground fault within 16 milliseconds or within the time required for a single 60 Hz sine wave). The GFCI according to the present invention should utilize an electronic switching component instead of an electro-mechanical device, but should not utilize a thyristor or like device which cannot switch instantaneously. The failure of any single element within the GFCI should not prevent the GFCI from properly functioning to detect and clear a ground fault, and preferably the GFCI should store the fault sequence and require resetting before a connection is permitted to be re-established. In addition, preferably the GFCI should have a self-protecting overcurrent trip circuit and should not require the use of a separate ground return wire. Further, the GFCI should have a self-testing circuit that can be utilized to simulate a fault and test the functioning of the GFCI and should include an audible and/or visual fault indicating alarm.
With the foregoing in mind, a primary object of the present invention is to provide an improved ground fault circuit interrupter which is highly reliable even in the event of a single component failure and which utilizes electronic sensing and switching components capable of detecting and clearing a ground fault preferably within 16 milliseconds or within the time required for a single 60 Hz sine wave.
Another object of the present invention is to provide a ground fault circuit interrupter which stores fault sequences and requires resetting before a connection is permitted to be re-established, which has a self-protecting overcurrent trip circuit, which detects faults from both the hot and neutral lines, and which does not require the use of a separate ground return wire.
A further object of the present invention is to provide a ground fault circuit interrupter which has a self-testing circuit that can be utilized to simulate a ground fault and test the functioning of the GFCI and which includes an audible and/or visual fault indicating alarm.
A still further object of the present invention is to provide a novel ground fault circuit interrupter and method which surpasses present industrial specifications defining adequate GFCI speed and reliability of operation and which provides individuals superior protection against injury from ground faults.
More specifically, the present invention is a ground fault circuit interrupter for use on a circuit including an AC power source and a load electrically connected via hot and neutral lines. The ground fault circuit interrupter has at least one switching component that forms a part of the hot line of the circuit for opening the circuit upon the occurrence of a ground fault. The switching component is an electronic semiconductor which permits substantially instantaneous switching. Preferably, the electronic semiconductor switching component is a MOSFET.
Preferably, the ground fault circuit interrupter has dual redundant channels each capable of detecting and opening the circuit should a ground fault occur. Thus, any one component failure within the ground fault circuit interrupter should not prevent the ground fault circuit interrupter from properly functioning to detect and clear a ground fault.
Preferably, each channel of the ground fault interrupter has a pair of MOSFET switching components each forming a part of the AC power source/load circuit on the hot line of the circuit. Each of the pair of MOSFET switching components is connected in series across its drain and source to the hot line of the circuit, and the sources of the pair of MOSFET switching components are arranged adjacent and back to back. Preferably each MOSFET switching component has an internal substrate diode connected across its drain and source, and the diodes are disposed back-to-back. Whereby, during a normal non-fault condition, current flows in a path from drain to source through one of the MOSFET switching components while current flows in a path from source to drain of the opposite MOSFET switching component through its respective diode, and during a ground fault condition, both MOSFET switching components are driven into an off condition and prevent any flow of current through the drain to source paths.
According to another aspect of the present invention, a method is provided for ground fault protecting a circuit having an AC power source and a load electrically connected via hot and neutral lines. The method includes placing each of a pair of MOSFETs in series across its drain and source to the hot line of the circuit such that the sources of the MOSFETs are disposed back to back. In addition, each MOSFET preferably has an internal substrate diode connected across its drain and source such that the diodes are disposed back-to-back. During a normal non-ground fault operating condition, current flows in a path from drain to source through one of the MOSFETs while current flows in a path from source to drain of the opposite MOSFET via its diode. When a ground fault is detected, both MOSFETs are substantially instantaneously driven into an off condition to prevent any flow of current within the MOSFETs through the drain to source paths. The use of the MOSFETs enable a ground fault to be detected and cleared within 16 milliseconds after initial occurrence of the ground fault and within one single AC fault current sine wave length of a 60 Hz sine wave.